Another post about a book I am reading, From Gestalt Theory to Image Analysis. I want to write a few paragraphs about another interesting idea I found in the book.

Two Gestalt laws can be used to explain some optical illusions.

The amodal completion law: “[W]hen a curve stops another curve, thus creating a “T-junction”… our perception tends to interpret the interrupted curve as the boundary of some object undergoing occlusion.” This law is also related to the good continuation law.

Penrose triangle and fork are illusions (confusions?) are caused by the perceived depth in the image, locally:

The perspective law: “Whenever several concurring lines appear in an image, the meeting point is perceived as a vanishing point (point of infinity) in a 3-D scene. The concurring lines are then perceived as parallel lines in space.” (Sounds reasonable, but how come all parallel lines are man-made?)

The Sander illusion (the left diagonal appears longer than the right one) and the Müller-Lyer illusion (the middle arrow appears longer) are caused by the perceived depth in the image:

I’d also add the Ponzo illusion (the “farther” bar appears longer than the “closer” one):

Also, remember Willy Wonka’s door?..

To summarize, both laws state that a person always sees 3D in a 2D image. But the fact is, one 2D image may correspond to many different 3D situations – including the drawing itself! That’s what causes the illusions.

So, these are interesting ideas that provide excellent explanations for the illusions. However, is it a good idea to try to design a computer vision system based on these laws? You don’t want to rely on a system that is so easy to fool…